//x*a==m+n*a
//y*b==m+n*b
#include<iostream>
using namespace std;
int main(){
  double x,a,y,b;
  cin>>x>>a>>y>>b;
  double n=(y*b-x*a)/(b-a);
  cout.precision(2);
  cout<<fixed;
  cout<<n;
}